In the recent years, ink jet printers are marketed at a reasonable price and widely used in printing on a variety of media. Paper sheets specialized for use with ink jet printers are commercially available in order that the ink jet printed matter be endowed with better properties including sharpness and gloss. The paper sheets specialized for use with ink jet printers have a smooth ink receiving layer on a surface of paper substrate so that the ink may develop its color in an aesthetic manner without bleeding. To this end, the paper substrate supporting the ink receiving layer is moderately hygroscopic. Then the paper substrate must be relatively thick because paper generally has the nature that on moisture absorption, it stretches and contracts, resulting in cockles.
However, thick paper sheets are bulky in volume and add to the cost. When piled up, thick paper sheets tend to come in close contact to increase the risk that two or more paper sheets are simultaneously fed from the feeder to the printer. The problem may be solved by impregnating paper with a resin. However, there is available no resin that has both moisture absorption and contraction resistance. For example, when paper is impregnated with an epoxy resin, good contraction resistance is endowed at the sacrifice of moisture absorption. When hygroscopic polymers such as polyvinyl alcohol are used for impregnation, good moisture absorption is obtained with no improvement in contraction resistance. Another approach is to blend cellulose fibers with other fibers such as glass fibers, but at the risk of degrading good printability inherent to paper.
JP-A 2000-52641 discloses a low contractible printing paper sheet which has been treated with a specific silane hydrolyzate. This treatment is effective for improving stretching/contracting properties. However, this treatment as such is difficult to improve print quality, requiring the additional step of coating with another resin.